Consolidated Statements Of Changes In Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Capital In Excess Of Par Value [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]
Beginning balance at Dec. 31, 2014		$ 1,667	$ 834,615	$ 420,226	$ (8,895)
Beginning balance, shares at Dec. 31, 2014		166,688
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Purchase and retirement of common stock		$ (60)	(22,984)	(266,543)
Purchase and retirement of common stock, shares		(5,951)
Issuance of common stock under the employee stock purchase plan		$ 1	2,798
Issuance of common stock under the employee stock purchase plan, shares		69
Issuance of common stock upon exercise of stock options		$ 29	62,716
Issuance of common stock upon exercise of stock options, shares	2,927	2,927
Stock-based compensation	$ (17,312)		17,312
Tax benefit on stock options exercised			16,056
Net income	331,655			331,655
Dividends declared ($0.58, $0.54 and $0.50 per share)	(82,478)			(82,478)
Other comprehensive gain (loss)	(16,395)				(16,395)
Total Equity at Dec. 31, 2015	$ 1,289,720	$ 1,637	910,513	402,860	(25,290)
Ending balance, shares at Dec. 31, 2015		163,733
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect upon adoption of ASU 2016-09			0	0
Purchase and retirement of common stock		$ (66)	(28,306)	(266,002)
Purchase and retirement of common stock, shares		(6,600)
Issuance of common stock under the employee stock purchase plan		$ 1	3,357
Issuance of common stock under the employee stock purchase plan, shares		88
Issuance of common stock upon exercise of stock options		$ 18	44,896
Issuance of common stock upon exercise of stock options, shares	1,809	1,810
Stock-based compensation	$ (16,017)		16,017
Tax benefit on stock options exercised			8,984
Net income	333,817			333,817
Dividends declared ($0.58, $0.54 and $0.50 per share)	(86,657)			(86,657)
Other comprehensive gain (loss)	(12,665)				(12,665)
Total Equity at Dec. 31, 2016	$ 1,303,114	$ 1,590	955,461	384,018	(37,955)
Ending balance, shares at Dec. 31, 2016	159,031	159,031
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect upon adoption of ASU 2016-09			0	0
Purchase and retirement of common stock		$ (44)	(20,243)	(227,827)
Purchase and retirement of common stock, shares		(4,403)
Issuance of common stock under the employee stock purchase plan		$ 1	3,280
Issuance of common stock under the employee stock purchase plan, shares		71
Issuance of common stock upon exercise of stock options		$ 24	50,263
Issuance of common stock upon exercise of stock options, shares	2,370	2,370
Stock-based compensation	$ (36,366)		36,366
Tax benefit on stock options exercised			0
Net income	404,389			404,389
Dividends declared ($0.58, $0.54 and $0.50 per share)	(91,444)			(91,444)
Other comprehensive gain (loss)	18,047				18,047
Total Equity at Dec. 31, 2017	$ 1,476,839	$ 1,571	1,027,709	467,467	$ (19,908)
Ending balance, shares at Dec. 31, 2017	157,069	157,069
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect upon adoption of ASU 2016-09			$ 2,582	$ (1,669)